Question: 3 pencils cost $4.29. Which equation would help determine the cost of 7 pencils?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 3 pencils. We want to know the cost of 7 pencils. We can write the numbers of pencils as a proportion: $\dfrac{3}{7}$ We know 3 pencils costs $4.29. We can let $x$ represent the unknown cost of 7 pencils. The proportion of these costs can be expressed as: $\dfrac{\$4.29}{x}$ The cost changes along with the number of pencils purchased, and so the two proportions are equivalent. $\dfrac{3}{7} = \dfrac{\$4.29}{x}$